The invention relates to a surface acoustic wave (SAW) device and, more specifically, to a device having a quartz crystal substrate with a predetermined crystalline orientation for causing a surface acoustic wave to propagate along a predetermined crystalline axis of the substrate.
This invention relates to an optimal surface acoustic wave orientation on single crystal silicon dioxide or SiO.sub.2, commonly referred to as quartz. By way of example, SAW devices are currently used as bandpass filters, resonators, delay lines, and convolvers, in a broad range of RF and IF applications such as wireless, cellular communication, and cable TV. Any piezoelectric crystal may be used to produce substrates for the construction of SAW devices. However, three piezoelectric crystals have become dominant in the SAW industry. They are lithium niobate, lithium tantalate, and quartz. There are several material properties that determine the usefulness of any particular substrate. These properties include: (1) SAW velocity, V.sub.SAW ; (2) SAW piezoelectric coupling coefficient, k.sup.2 ; (3) power flow angle, PFA; (4) diffraction or beam spreading coefficient, gamma (.gamma.); and (5) temperature coefficient of delay, TCD. It is rare to find an orientation of a crystal to produce a substrate which optimizes these properties at the same time. Therefore, the choice of the crystal and orientation depends on desirable features for the specific application, and most always involves a compromise between these SAW material properties. By way of example, a high velocity is desirable for high frequency devices, because the device geometry patterns are larger and, therefore, the devices are easier to fabricate. At low frequencies, a low velocity is usually desirable because the device size is smaller, resulting in lower device and packaging costs. Thus, there is no universally optimum velocity. Likewise, the desired value for k.sup.2 is often set by the desired bandwidth of the SAW device. Large values of k.sup.2 are well suited for broad bandwidth devices and lower values of k.sup.2 for narrow bandwidth devices. Quartz is well suited for narrow bandwidth applications, lithium niobate for broad bandwidths, and lithium tantalate for moderate bandwidths. For most devices, particularly narrow band devices, the TCD should be as low as possible, and ideally zero. From this point of view, ST-quartz is best, lithium niobate is worst, and lithium tantalate is in between. The optimal value of gamma is -1.0, which results in minimum beam spreading. From this point of view, YZ lithium niobate is preferred as ideal, ST-quartz is worst, and lithium tantalate is in between. The PFA should be zero, and this is the case for most of the commonly used SAW substrates, with an exception being the 112 lithium tantalate, which has a PFA of 1.55 degrees. For narrow band applications, ST-quartz is the standard choice. However, narrow bandwidth SAW devices are generally constructed using long transducers, which enhance the beam spreading nature of the ST-quartz substrate. What is needed is a quartz substrate orientation that offers a TCD, k.sup.2, and PFA which are essentially the same as those for ST-quartz, and at the same time a diffraction coefficient, gamma, at or near the optimal value of -1.0.
The present invention satisfies this need and provides a different orientation of quartz which has far superior properties to ST-quartz, in that, the substantial beam spreading nature of ST-quartz (gamma=+0.38) has been dramatically reduced (gamma =-1.0).